[vm] Enable non-nullable experiment in the VM by default

VM was not performing auto-detection of sound null safety mode
if --enable-experiment=non-nullable was not specified on the command
line. This change flips that logic to perform auto-detection unless
--enable-experiment=no-non-nullable is specified.

TEST=Added regression test runtime/tests/vm/dart/regress_44026_test.dart.

Fixes https://github.com/dart-lang/sdk/issues/44026
Fixes https://github.com/dart-lang/sdk/issues/44043

Change-Id: Ide8935e90dd3a8bd5a417aeb758605dedbb853eb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/170201
Commit-Queue: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Siva Annamalai <asiva@google.com>

runtime/tests/vm/dart/regress_44026_main.dart

@@ -0,0 +1,12 @@
+// Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Main script for regress_44026_test.dart.
+// @dart=2.12
+
+import 'regress_44026_opt_out_lib.dart';
+
+void main() {
+  sayHello(true);
+}

runtime/tests/vm/dart/regress_44026_opt_out_lib.dart

@@ -0,0 +1,14 @@
+// Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Opted-out library for regress_44026_test.dart.
+// @dart = 2.7
+
+void sayHello(bool t) {
+  String s;
+  if (t) {
+    s = 'hello';
+  }
+  print(s);
+}

runtime/tests/vm/dart/regress_44026_test.dart

@@ -0,0 +1,44 @@
+// Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// OtherResources=regress_44026_main.dart
+// OtherResources=regress_44026_opt_out_lib.dart
+
+// Tests that compile-time error is issued if NNBD opted-out library is used
+// from opted-in entry point (with null safety auto-detection).
+// Regression test for https://github.com/dart-lang/sdk/issues/44026.
+
+import 'dart:io' show File, Platform, Process;
+
+import 'package:expect/expect.dart';
+import 'package:path/path.dart' as path;
+import 'snapshot_test_helper.dart';
+
+const int kCompilationErrorExitCode = 254;
+
+main() async {
+  await withTempDir((String temp) async {
+    // Need to copy test scripts out of Dart SDK to avoid hardcoded
+    // opted-in/opted-out status for Dart SDK tests.
+    for (String script in [
+      'regress_44026_main.dart',
+      'regress_44026_opt_out_lib.dart'
+    ]) {
+      final scriptInTemp = path.join(temp, script);
+      File.fromUri(Platform.script.resolve(script)).copySync(scriptInTemp);
+    }
+
+    // Do not add Platform.executableArguments into arguments to avoid passing
+    // --sound-null-safety / --no-sound-null-safety arguments.
+    final result = await Process.run(Platform.executable, [
+      path.join(temp, 'regress_44026_main.dart'),
+    ]);
+    print('stdout: ${result.stdout}');
+    print('stderr: ${result.stderr}');
+    Expect.equals(kCompilationErrorExitCode, result.exitCode);
+    Expect.stringContainsInOrder(result.stderr, [
+      "Error: A library can't opt out of null safety by default, when using sound null safety."
+    ]);
+  });
+}

runtime/tests/vm/vm.status

@@ -256,6 +256,7 @@ dart/appjit*: SkipByDesign # Test needs to run from source
 dart/kernel_determinism_test: SkipByDesign # Test needs to run from source
 dart/minimal_kernel_test: SkipByDesign # Test needs to run from source
 dart/null_safety_autodetection_in_kernel_compiler_test: SkipByDesign # Test needs to run from source
+dart/regress_44026_test: SkipByDesign # Test needs to run from source
 dart/snapshot_depfile_test: SkipByDesign # Test needs to run from source
 dart/type_casts_with_null_safety_autodetection_test: SkipByDesign # Test needs to run from source
 dart_2/appjit*: SkipByDesign # Test needs to run from source

runtime/vm/dart.cc

@@ -806,7 +806,7 @@ bool Dart::DetectNullSafety(const char* script_uri,
   }
 
   // If we are loading from source, figure out the mode from the source.
-  if (KernelIsolate::GetExperimentalFlag("non-nullable")) {
+  if (!KernelIsolate::GetExperimentalFlag("no-non-nullable")) {
     return KernelIsolate::DetectNullSafety(script_uri, package_config,
                                            original_working_directory);
   }

tests/lib/isolate/detect_nullsafety_1_test.dart

@@ -21,8 +21,8 @@ void main() {
   try {
     // Running from Source.
     testNullSafetyMode(sourcePath, 'Strong Mode');
-    // Without the enable experiment option it will be in weak mode.
-    testNullSafetyMode1(sourcePath, 'Weak Mode');
+    // Without the enable experiment option it will be in strong mode.
+    testNullSafetyMode1(sourcePath, 'Strong Mode');
 
     // Running from Kernel File.
     testNullSafetyMode(dillPath, 'Strong Mode');